The present invention relates to a high-pressure sodium lamp, particularly to a configuration of a discharge tube in a high-pressure sodium lamp with a high color rendering property.
FIG. 3 shows an example of the configuration of a discharge tube in a conventional high-pressure sodium lamp. As shown in FIG. 3, this conventional high-pressure sodium lamp includes a conductive tube 33 and an electrode 32 held at one end of the conductive tube 33 using titanium solder 31. The other end of the conductive tube 33 is an open end.
The conductive tube 33 is attached to one end of a transparent alumina tube 34, and a portion of the transparent alumina tube 34 to which the conductive tube 33 is attached is sealed hermetically with a sealer 35 made of ceramic cement. Sodium amalgam 36 is provided at an inner end of the transparent alumina tube 34.
When using the above-mentioned conventional configuration of the discharge tube, however, in a high-pressure sodium lamp with a relatively high sodium-vapor pressure inside the discharge tube in operation, particularly, in a high-pressure sodium lamp with a high color rendering property, the difference in pressure between the inside and the outside of the transparent alumina tube 34 occurs during operation and the transparent alumina tube 34 comes to have a high temperature. As shown in FIG. 4, therefore, a portion in the vicinity of the electrode 32 in the conductive tube 33 might be deformed.
When such deformation occurs, the conductive tube 33 comes off from the sealer 35, thus forming a gap between them. Into this gap, the sodium amalgam 36 intrudes and therefore the sodium of a luminescent material reacts with the sealer 35 over a wide area. Consequently, the loss of the sodium is promoted inside the discharge tube, thus causing problems such as the variation in discharging color or in lamp voltage during the lifetime and the like in some cases.
Therefore, as an example of a configuration for solving such problems, JP 8-399 B discloses a high-pressure sodium lamp in which a conductive tube is prevented from being affected by the difference in pressure between the inside and the outside of a discharge tube and ceramic cement is prevented from being exposed in a discharge space of the discharge tube, thus suppressing the reaction between sodium and the ceramic cement during operation.
However, in the above-mentioned high-pressure sodium lamp disclosed in JP 8-3995 B, sodium amalgam of a luminescent material is maintained not inside the discharge tube but inside the conductive tube, which is the coldest portion, thus causing the following two problems.
The first problem is that heat generated by an arc discharge between electrodes serving as a heat source in operation is intercepted by the electrodes, and the sodium amalgam maintained inside the conductive tube cannot receive the heat easily, thus requiring a long time to reach a stable lighting state after turning on the lamp.
The second problem is that when the conductive tube is displaced in being attached to the discharge tube, the temperature of the coldest portion varies, thus increasing the variation in lamp voltage of manufactured lamp compared to the case where sodium amalgam is provided at the inner end of a discharge tube where the temperature does not vary greatly as shown in FIG. 3.
JP 52-42673 A discloses an example in which a conductive tube has an airtight structure. In the conductive tube, the open end of the conductive tube 33 shown in FIG. 3 is closed and the inside of the conductive tube is shielded from a gaseous substance surrounding the inside of a discharge tube. Thus, the reaction between the portion to be a high temperature in the conductive tube 33 and the gaseous substance surrounding it is prevented. JP 52-42673 A describes the simple shielding but no measures against the deformation caused by a difference in pressure.
The present inventors operated high-pressure sodium lamps with a high color rendering property manufactured to have discharge tubes as shown in FIG. 3 for about 6000 hours and checked a loss amount of sodium in the discharge tubes with a deformed conductive tube and with a non-deformed conductive tube, respectively. As a result, in the discharge tube with a deformed conductive tube, about 50% of the total amount of sodium sealed in the discharge tube was lost. On the other hand, in the discharge tube with a non-deformed conductive tube, about 4% of the total amount of sodium was lost. Thus, it was confirmed that the loss amount in the discharge tube with a non-deformed conductive tube is extremely small compared to that in the discharge tube with a deformed conductive tube.
In addition, the discharge tube with a deformed conductive tube was checked in detail. As a result, about 90% of the loss amount of sodium was caused by the reaction between the sodium and the sealer due to the gap formed by the coming off of the conductive tube from the sealer. In other words, it was confirmed that the reaction between the sodium and the sealer can be suppressed by preventing the conductive tube from being deformed.
The present invention is intended to provide a high-pressure sodium lamp in which the conductive tube is prevented from being deformed and sodium is provided inside a discharge tube as a luminescent material, whereby a lighting color and lamp voltage are prevented from varying during the lifetime, the time required for reaching a stable lighting state after turning on the lamp is short, and the variation in lamp voltage is suppressed.
In order to achieve the above-mentioned object, a high-pressure sodium lamp of the present invention includes a discharge tube and a pair of electrodes opposing each other inside the discharge tube, and at least sodium and a noble gas are sealed in the discharge tube. The pair of electrodes are held by conductive tubes attached hermetically to both ends of the discharge tube with a sealer, and the conductive tubes have airtight structures and an inert gas is sealed therein.
According to this configuration, in operation, due to the pressure of the inert gas sealed in the conductive tubes, the difference in pressure between portions located inside and outside the discharge tube in the conductive tubes is not caused easily. Further, the heat conduction by the inert gas sealed in the conductive tubes lowers the temperature of portions in the vicinities of the electrodes in the conductive tubes. As a result, the conductive tubes can be prevented from being deformed and coming off from the sealer.
Moreover, since the sodium is sealed in the discharge tube, the sodium of a luminescent material can receive quickly the heat generated by an arc discharge between the electrodes serving as a heat source in operation, and at the same time, the temperature is kept constant. Thus, the time required for reaching a stable lighting state after turning on the lamp is shortened and the variation in lamp voltage during manufacture can be suppressed.
In the above-mentioned high-pressure sodium lamp, it is preferable that the pressure of the inert gas sealed in the conductive tubes is at least 10 Torr.
According to this configuration, the difference in pressure between the portions of the conductive tubes located inside and outside the discharge tube is further reduced, thus more reliably preventing the conductive tubes from being deformed and coming off from the sealer.
In the above-mentioned high-pressure sodium lamp, it is preferable that the portions holding the electrodes in the conductive tubes have a temperature of 800xc2x0 C. or lower.
According to this configuration, the load on the conductive tubes according to the temperature is suppressed, thus further reliably preventing the conductive tubes from being deformed and coming off from the sealer.